He said the finding of a sharp drop in the minnow population attributed only to synthetic estrogen was surprising.

He had suspected that feminization in addition to other environmental stresses could lead to population collapse. The new study shows feminization alone causes the decline.

Study leader Kidd and colleagues believe that if they had continued to add estrogen to the experimental lake for more than three years, the estrogen would also have had an impact on the populations of longer-lived fish in the lake—trout, white suckers, and pearl dace.

"It's critical that our municipal wastewaters are treated at least with secondary treatment to reduce the input of these estrogens into surface waters," Kidd said.